For contacts:
- Export contacts from Incredimail to a .csv file (File | Export...)
- Import the .csv file into the address book in Thunderbird (Tools | Import...)

For e-mails:
- You may be able to use IncrediConvert (free) to convert messages to .eml format (but see the caveat).
- Import the .eml files into Outlook Express.
- Import from Outlook Express into Thunderbird.